Re: [PATCH] Remove blkg node after destroying blkg

[Tao Su]

On Tue, Apr 25, 2023 at 04:09:34PM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2023/04/25 15:59, Tao Su 写道:
> > Kernel hang when poweroff or reboot, due to infinite restart in function
> > blkg_destroy_all. It will goto restart label when a batch of blkgs are
> > destroyed, but not remove blkg node in blkg_list. So the blkg_list is
> > same in every 'restart' and result in kernel hang.
> > 
> > By adding list_del to remove blkg node after destroying, can solve this
> > kernel hang issue and satisfy the previous will to 'restart'.
> > 
> > Reported-by: Xiangfei Ma <xiangfeix.ma@xxxxxxxxx>
> > Tested-by: Xiangfei Ma <xiangfeix.ma@xxxxxxxxx>
> > Tested-by: Farrah Chen <farrah.chen@xxxxxxxxx>
> > Signed-off-by: Tao Su <tao1.su@xxxxxxxxxxxxxxx>
> > ---
> >   block/blk-cgroup.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> > index bd50b55bdb61..960eb538a704 100644
> > --- a/block/blk-cgroup.c
> > +++ b/block/blk-cgroup.c
> > @@ -530,6 +530,7 @@ static void blkg_destroy_all(struct gendisk *disk)
> >   		spin_lock(&blkcg->lock);
> >   		blkg_destroy(blkg);
> > +		list_del(&blkg->q_node);
> 
> blkg should stay on the queue list until blkg_free_workfn(), otherwise
> parent blkg can be freed before child, which will cause some known
> issue.

Yes, directly removing blkg node is not appropriate, which I noticed some
comments in blkg_destroy(), thanks for pointing out this issue.

> 
> I think this hung happens when total blkg is greater than
> BLKG_DESTROY_BATCH_SIZE, right?

Yes, you are right.

> 
> Can you try if following patch fix your problem?

This patch can also fix my problem, and indeed is a more secure way.

Thanks,
Tao

> 
> index 1c1ebeb51003..0ecb4cce8af2 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -527,6 +527,9 @@ static void blkg_destroy_all(struct gendisk *disk)
>         list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
>                 struct blkcg *blkcg = blkg->blkcg;
> 
> +               if (hlist_unhashed(&blkg->blkcg_node))
> +                       continue;
> +
>                 spin_lock(&blkcg->lock);
>                 blkg_destroy(blkg);
>                 spin_unlock(&blkcg->lock);
> 
> >   		spin_unlock(&blkcg->lock);
> >   		/*
> >